The present invention relates to an electronic musical instrument and, more particularly, to a digital tone generating system suitable for the large scale integrated circuit (hereinafter referred to as an LSI circuit).
Conventionally, many kinds of proposals concerning digital tone source circuits for electronic musical instruments have often been tried. The complex waves including many harmonics have been read in wave data with a given clock from a read only memory (hereinafter referred to as an ROM) or from a random access read/write memory (hereinafter referred to as a RAM) to provide the tone wave. Thereafter, the given envelope has been attached to the tone wave by a digital technique or an analog technique to thereby provide the tone signal.
Some problems which occur in such a case are as follows. As a first problem, a calculation for making the waves has existed. Since to change the tone color, the complex waves are changed in shape within this instrument, when the tone color data are provided at the proportions of each of the harmonics, like the draw-bar most used for the electronic musical instrument, in the order of the level of the 8 feet (fundamental), the level of the 4 feet (second harmonics) and the level of the 22/3 feet (third harmonics), the complex wave corresponding in shape to it has to be made from the tone color data. Namely, an inverted fourier transform is required to be performed. Although recently, microcomputers are available at lower cost, the inverted fourier transform requires a computing time of from several hundreds of milliseconds to approximately one second. In addition, the inverted fourier transform is required to be performed every time a player changes drawbars or switches tone tablets. Thus, when more time is required for calculation, the tone color may not change immediately or the tone may not be made for some time. Accordingly, these problems are not suitable for the performance of the musical setting which often requires frequent color-tone switching.
As a second problem, the tone color remains unchanged from the time for the tone to be made to the time for the tone to be disappeared. If the inverted fourier transform is performed from the tone color data and the wave data is provided, the wave data is written in the memory and the wave data of the memory is repeatedly read at a given clock rate, with the result that the wave normally becomes constant. Even if a given envelope is attached to the wave, the tone color remains unchanged. To change the tone color every moment, the memory wave is required to be rewritten every moment. Since the memory itself is normally read, it is required to be written into between the read timings in a synchronous relationship with the read cycle for the rewriting of the memory contents. The read clock is not always constant, since it changes with the produced step, and it is very difficult to rewrite the waves in terms of hardware. As described hereinabove, the tone-color change means a high-speed inverted fourier transform for each moment, since the inverted fourier transform is required to be performed each time from the tone color data to provide the wave data. Even from this point, it can be apparent that the tone color is extremely difficult to be changed every moment.
As a third problem, there is a problem of the system clock of the whole hardware. The digital circuit is adapted to operate under a fixed clock for an easier synchronous relationship of the whole system, whereby the timing between the logic circuits is rendered definite and the construction of the hardware is rendered simpler. On the other hand, in the tone source circuit of the electronic musical instrument, twelve different clocks are provided to obtain the tone signal of each note of C, C.sup..music-sharp., D . . . B so as to thereby change the read speed. For instance, to change the octave in the order of C.sub.1, C.sub.2, C.sub.3 . . . , the clock for C note is required to be rendered 1/2, 1/4, 1/8 . . . , or the memory address is required to be read by 2 jumps, 4 jumps, 8 jumps, . . . . However, the clock of the C.sup..music-sharp. note is required to be 2 .sup.1/12 times as fast as the clock of the C note. Similarly, the clock of the D note is required to be 2 .sup.1/12 times as fast as the clock of the C note. The clock of the D.sup..music-sharp. note is required to be 2 .sup.1/12 times as fast as the clock of the C note. Since these 2.sup.1/12, 2.sup.2/12, 2.sup.3/12, . . . are irrational numbers, 12 independent clock generators are required to be disposed to generate these 12 clocks by the hardware. The problem is that a synchronous relationship cannot be provided, and the hardware cannot be commonly used, since the twelve clock speeds are completely independent. Accordingly, since a plurality of envelope multipliers and a plurality of digital-to-analog converters (hereinafter referred to as D/A converters) are required, the hardware becomes extremely large in scale, thus resulting in a complicated system construction.